chore(inputs.jolokia2): Move agent and proxy one level up

[srebhan]

• [x] Updated associated README.md.
• [x] Wrote appropriate unit tests.
• [x] Pull request title or commits are in conventional commit format

This PR moves the jolokia2_agent and jolokia2_proxy input plugins from the plugins/inputs/jolokia2 directory on level up to plugins/inputs. This way, the two plugins are adhering now to the standard structure, making discovery easier. The common parts of the two plugins moved to plugins/common/jolokia2 and the READMEs, registrations, tests, etc were split accordingly.

[reimda]

Thanks for moving these into plugins/inputs. It will be easier for everyone to find them when they're not nested under plugins/inputs/jolokia2

[telegraf-tiger[bot]]

Download PR build artifacts for linux_amd64.tar.gz, darwin_amd64.tar.gz, and windows_amd64.zip. Downloads for additional architectures and packages are available below.

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Artifact URLs

DEB	RPM	TAR GZ	ZIP
amd64.deb	aarch64.rpm	darwin_amd64.tar.gz	windows_amd64.zip
arm64.deb	armel.rpm	darwin_arm64.tar.gz	windows_i386.zip
armel.deb	armv6hl.rpm	freebsd_amd64.tar.gz
armhf.deb	i386.rpm	freebsd_armv7.tar.gz
i386.deb	ppc64le.rpm	freebsd_i386.tar.gz
mips.deb	riscv64.rpm	linux_amd64.tar.gz
mipsel.deb	s390x.rpm	linux_arm64.tar.gz
ppc64el.deb	x86_64.rpm	linux_armel.tar.gz
riscv64.deb		linux_armhf.tar.gz
s390x.deb		linux_i386.tar.gz
		linux_mips.tar.gz
		linux_mipsel.tar.gz
		linux_ppc64le.tar.gz
		linux_riscv64.tar.gz
		linux_s390x.tar.gz
		static_linux_amd64.tar.gz